The invention relates to a process for the production of a sheet metal which has recrystallized as fine grains and is suitable for superplastic forming from a work-hardenable, age-hardenable aluminum alloy.
In the case of age-hardenable aluminum alloys, an increase in strength can also be achieved by heat treatment as well as by cold forming. These alloys, for example of the AlZnMg and AlZnMgCu type, tend to grain formation if the solution annealing necessary for precipitation hardening is associated with a recrystallization. However, for numerous applications, in particular for superplastic forming, a fine-grained character is desirable or a prerequisite. In the case of sheet metal, for example, which is to be subjected to superplastic forming, the grain size is below 25 .mu.m and preferably below 10 .mu.m. In addition, the grains should be in virtually globulitic form. Moreover, the substantial increase in size of the regularly distributed grains or sub-grains must also not take place during superplastic forming, which is carried out at or just above 500.degree. C.
The plastic extension of a superplastic aluminum alloy is usually in the range of 400-800%, that is to say far above the values for conventional alloys. This allows multiple possibilities for shaping in respect of function and design with economic production from one piece. The variety, of shapes are reproducible with high dimensional accuracy; no "spring-back" occurs. The simple tools which can be used and which permit even small and medium production runs at favorable costs and can be produced with short delivery times are turned to particular advantage. Changes in shape can be carried out rapidly at acceptable cost.
Numerous binary and ternary aluminum alloys having superplastic characteristics have been described, in particular also of the AlMg type, for example in EP,A1 0297035.